5203 L $$aThe administration of 13-cis retinoic acid (13cisRA) improves 3-year event free survival rates in children with high-risk neuroblastoma. However, more than half of patients suffer relapse within 3 years. A high degree of variability in UcisRA pharmacokinetics has been reported, suggesting that individualisation of dosing may further improve therapy. Several cytochrome P450 (CYP) and UDPglucuronosyltransferase (UGT) enzymes play a role in the metabolism of retinoic icid. Thus, genetic variation in CYP and UGT activity could underlie the individual differences in 13c/5RA metabolism and bioavailability.